1. Field of the Invention
This invention relates generally to a manufacturing process for a compressed hydrogen tank and, more particularly, to a manufacturing process for a compressed hydrogen tank that includes injection molding a portion of a liner of the tank to fill a sealing gap in a tank boss and then rotomolding the rest of the liner thereafter.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. The automotive industry expends significant resources in the development of hydrogen fuel cell systems as a source of power for vehicles. Such vehicles would be more efficient and generate fewer emissions than today's vehicles employing internal combustion engines.
Typically hydrogen is stored in a compressed gas tank under high pressure on the vehicle to provide the hydrogen necessary for the fuel cell system. The pressure in the compressed tank can be upwards of 700 bar. In one known design, the compressed tank includes an inner plastic liner that provides a gas tight seal for the hydrogen, and an outer carbon fiber composite layer that provides the structural integrity of the tank. Because hydrogen is a very light and diffusive gas, the inner liner and the tank connector components, such as O-rings, must be carefully engineered in order to prevent leaks. The hydrogen is removed from the tank through a pipe. At least one pressure regulator is typically provided that reduces the pressure of the hydrogen within the tank to a pressure suitable for the fuel cell system.
As the hydrogen is removed from the compressed tank, the pressure of the hydrogen in the tank will decrease. When the pressure of a gas is reduced and the volume does not change, the temperature of the gas will also decrease. The effect of the decrease in the temperature will be limited because heat is transferred from the environment into the tank. If the flow rate of the hydrogen flowing out of the tank is high enough and/or the temperature of the environment is low enough, the temperature in the tank can fall below −80° C. Typically it is possible to limit the hydrogen flow rate so that −80° C. is the lowest temperature that occurs within the tank. Further, when the tank is being filled with hydrogen, the temperature of the hydrogen can increase to 80° C. due to the compression of the hydrogen inside of the tank, providing a temperature swing of −80° C. to 80° C. The materials that can seal hydrogen in this temperature range are difficult to produce.
If the temperature of the hydrogen within the tank decreases beyond a certain temperature, such as −80° C. around the liner and −40° C. at the tank seals, including O-rings and other sealings, the materials become brittle and possibly damaged, affecting the tank's gas tight performance. Therefore, there are limits as to how fast hydrogen and/or for how long hydrogen can be removed from the compressed tank in a fuel cell system.
FIG. 1 is a cut-away, cross-sectional view of a known compressed hydrogen storage tank 10 of the type discussed above. The tank 10 includes an outer structural layer 12, typically made of a graphite composite, and an inner liner 14, typically made of a durable plastic, such as a high density polyethylene. The liner 14 provides the gas tight environment for the hydrogen gas, and the outer layer 12 provides the structural integrity for the compressed hydrogen gas. An opening 16 in the outer layer 12 provides a location where hydrogen is removed from the tank 10 and put into the tank 10. An annular neck portion 18 of the liner 14 extends partially into the opening 16. A boss 20 is provided within the opening 16 and is formed around the neck portion 18 between the liner 14 and the outer layer 12. The boss 20 is typically made of metal, such as stainless steel, and has a configuration for a particular tank design.
A bore 22 extending through the boss 16 accepts an in-tank valve block 24. The valve block 24 includes all of the necessary components that are required for the operation of the tank system. An O-ring 30 is provided between the valve block 24 and the boss 20 to provide seal integrity. This configuration of the tank 10 allows the hydrogen to be removed from the tank 10 through line 32 without loss of seal integrity.
The tank 10 is an improvement over known compressed hydrogen tanks that are complicated and require several O-rings. The design of the tank 10 is intended to prevent hydrogen from leaking through the opening 16 and the outer layer 12 at lower pressures. Particularly, the boss 20 includes an annular channel 34 that defines an edge flange 36. The channel 34 maintains a tighter seal between the boss 20 and the liner 14 that helps prevent gas leaks at lower pressures. This design eliminates most of the O-rings in the valve block 24 that were required in conventional compressed hydrogen tanks.
The most cost effective technique for molding the plastic liner 14 is by a rotomolding process, well known to those skilled in the art. In this process, the plastic liner material is applied as a granulate to a mold shaped like the liner 14. The mold is rotated and heated so that the plastic material is evenly distributed on the mold surface to form the liner 14. The boss 20 is positioned in the mold during the rotomolding process so that the plastic liner material is formed around the neck portion 18 of the liner 14, as shown in FIG. 1. Once the boss 20 is formed to the liner 14, the valve block 24 can be threaded to the boss 20 to provide the seal integrity.
The rotomolding process is not conducive for filling small gaps, and therefore the channel 34 may not completely fill with the liner material during the rotomolding process, thus affecting the sealing properties at this location in the tank 10. Therefore, known designs that employ the channel 34 require that the entire liner 14 be injection molded when it is fabricated. However, because of the size of the tank 10 and the time required to injection mold, injection molding the liner 14 is not a cost effective process.